Jim: Oddly, the build quality on the B.2600 is actually pretty decent. They even went the extra step of venting the metal case to get the temp-sensitive components to settle down easier. Fact is, though...the original's hardware was kind of "meh" anyway. You got left with two rather cumbersome lids to store, Tolex that wasn't always glued down properly, slider caps that seemed to want to hide along with the missing socks from your dryer, etc. And of course, that VCF swap, and the odd heat/cool issues that would affect tuning, drift, etc. But you can still field-recalibrate it if needed...and the service manual should be essentially identical to the original, which you can find HERE: https://www.vintagesynthparts.com/service-manual/ Just make sure to have a multimeter and o-scope handy.
Garfield: Yep, the Lehle filter is a perfect "fix" for the 2600's DC-coupled VCA. Like I said, this behavior is 1:1 with the original in all of its revisions. However, sometimes you can get the VCA to provide a useful subsonic "thump", and this works really nicely when programming percussive sounds, so being able to take the DC filter out of line is important. But again, note the reply to Jim above re: the service manual. The 2600 is easy enough to recalibrate, and recalibrating the ARP 2600 was something A LOT of synthesists would do to dial in the fine details of the sounds they were shooting for. I even got tasked with recalibrating MTSU's ARP several times when I was more or less "in charge" of all of the audio implementations in the School of Music, and it's NOT something super-involved and difficult. But if you want to tweak the VCA's behavior in general, the original service manual by ARP is just fine as a reference.